We have implemented a major longitudinal twin study of aggression and antisocial behavior (ASB) and its social and biological risk factors (#MH58354). Based on two waves of data collected at ages 9-10 (Wave 1) and 11-12 (Wave 2), we study genetic and environmental variations in childhood conduct problems, and how these vary across gender, rater, and definition of ASB. Relationships of ASB with known risk factors (social and biological) have been confirmed, and new relationships have been identified. The extent to which risk factor/ASB relationships are mediated by environmental and/or genetic factors is studied and is of key importance to future intervention. We propose to extend the study to include 2 additional waves of assessment at ages 14-15 (Wave 3) and 16-17 (Wave 4), to obtain comprehensive measures of antisocial and aggressive behavior and their risk factors for these same children during adolescence. The primary aim of the next phase is to understand environmental, genetic, and phenotypic continuities in ASB and its biological and social risk factors from age 9-17. The first 5-year phase provided comprehensive assessments of behavioral, neurocognitive, social, and psychophysiological function during two waves of investigation. In Wave 1, twins and their primary caregivers participated in the most extensive assessment, including structured interviews about behavior and psychosocial risk factors, neurocognitive testing, and psychophysiological responses (cardiovascular, electrodermal, electrocortical, and electromyographic) during rest periods and a variety of tasks (attention and emotion-related). A follow-up assessment in Wave 2 focused primarily on children's'behavior and social risk factors (peer and family characteristics), with a limited psychophysiological recording (autonomic measures only). Adding two more waves of data collection during adolescence will provide the most comprehensive, prospective longitudinal data on ASB ever obtained in a genetically informative design. This four-wave, 10 year longitudinal twin study will provide the unique opportunity to understand how genes and environment combine and interact to produce antisocial outcomes in children and adolescents. In this revised application, clarifications of theoretical framework and a detailed plan for analyses are provided. The data analysis core staff has been expanded to assist in the complex phenotypic and genetic analyses that will ensue from this rich and unique longitudinal dataset.